engine: remove host if it is already detached in gluster

  Handle the error returned from VDSM while removing a host. If
the host is already detached from Gluster through Gluster CLI then
remove the host from engine .

Change-Id: Ib83bffebe99ef5553ffcb2ce8ef03d4bba4c12b3
Bug-Url:i https://bugzilla.redhat.com/1024183
Signed-off-by: Ramesh Nachimuthu <rnachimu@redhat.com>